1. Field of the Invention
This invention relates to a fountain solution for lithographic offset printing presses and more particularly to a fountain solution that is free of highly volatile constituents such as isopropyl alcohol.
2. Description of the Prior Art
In the practice of lithographic printing, an aqueous fountain solution is used to maintain the non-image areas of the printing plate insensitive to ink. The typical fountain solution is made up from a fountain etch concentrate, water and from 10 to 30 percent by volume of isopropyl alcohol. The fountain etch concentrate includes an acidic component such as phosphoric acid or citric acid, buffering salts, water-soluble resins or gums such as gum arabic or cellulose gum and frequently a wetting agent. The fountain solution is generally acidic with a pH value between about 4.5 and 5.5. Printing problems sometimes occur when the fountain solution deviates significantly above or below this pH range.
Many lithographic presses have a fountain solution distribution system that is separate from the ink distribution system. Generally, the fountain solution distribution system includes a ductor roller which has intermittent or interrupted flow of the fountain solution from the reservoir to dampening form rollers that contact the printing plate. Often these conventional dampening systems use paper or molleton (cloth) covered rollers or specially treated rollers in the dampening system roller train to act as intermediate fountain solution reservoirs. Alternately brushes can flick droplets of water onto form rollers or directly onto the plate or nozzles can similarly spray a fine mist.
Historically, these systems did not require the use of isopropyl alcohol in the fountain solution to obtain competitively acceptable printing quality. A significant number of the printers presently add up to about 10 to 15 percent by volume of isopropyl alcohol to the fountain solutions to obtain better control of the ink and water feed.
Some lithographic presses are equipped with a continuous feed dampening system sold by Dahlgren Mfg. Co., Dallas, Tex., under the tradename Dahlgren. Other dampening systems of the direct continuous type include the system sold by Miehle-Goss-Dexter, Chicago, Ill., under the trademark Miehlematic, and by Harris Corp., Cleveland, Ohio, under the trademark Microflow and by Miller Western Mfg. Co., Pittsburg, Pa., under the trademark Millermatic.
In the Dahlgren system, the printing plate is contacted only by inked rollers, that is, the fountain solution must be carried from the dampening unit rollers by means of one or more inked rollers, usually one of the form rollers, to the printing plate. This type of system requires the assistance of a water transport additive such as a water soluble glycol as disclosed in U.S. Pat. No. 3,625,715 or an alcohol such as disclosed in U.S. Pat. No. 3,168,037, with isopropyl alcohol being almost universally used. The excellent and more independent control of ink and water delivery to the printing plate accounts for the ever increasing use of the Dahlgren system in lithographic printing. This, in turn, accounts for the extensive use of isopropyl alcohol in Dahlgren continuous dampening systems. Typically, the fountain solution will contain between about 15 to 30 percent isopropyl alcohol depending upon the specific press, speed, type of form and substrate being printed. The use of isopropyl alcohol is the best compromise between good press and printing performance and cost of the fountain solution.
Another variety of a continuous contact dampening system is the Millermatic type wherein the fountain solution is applied to the printing plate by means of a dampener form roller that is not part of the inking system. With such an arrangement it would be expected that isopropyl alcohol would not be required because the inked form roller is not used to distribute the aqueous fountain solution. Because, however, of the excellent ink and water balance control, it is also common to use isopropyl alcohol as a constituent in the dampening solution used with the Millermatic type of dampener.
The basis for isopropyl alcohols' utility in continuous lithographic systems resides partly in its stability to transport the fountain solution to the printing plate by means of the inked rollers. This property has been attributed to the low surface tension of the aqueous solutions at volume concentrations above about 10 percent isopropyl alcohol. This allows the fountain solution to wet and mix with the ink which normally can be done only with considerable difficulty. The enhanced wetting effect allows the water-containing fountain solution to be carried on or within the film of ink on the form roller and then to the printing plate where it perferentially deposits in the hydrophilic non-image areas.
The exact mechanism by which isopropyl alcohol achieves this advantageous effect is not completely understood within the industry. For this reason empirical data is not available to predict with any degree of certainity constituents which would serve as a replacement for isopropyl alcohol.
Because of the toxic and flammable properties of isopropyl alcohol and because it is relatively expensive, there have been suggestions of materials which would be utilized to replace the isopropyl alcohol in fountain solutions. In U.S. Pat. No. 4,030,417 a fountain solution formulation is disclosed in which two etch concentrates are prepared. The first has as its primary ingredients a fatty acid material, a monovalent hydroxide and water and the second has as its primary ingredients gum arabic, a monovalent iodide and water. The solutions are mixed to form an etch concentrate which is thereafter mixed with water to produce the final fountain solution formulation. The fatty acid material is disclosed as a higher fatty acid having at least six carbon atoms in a linear chain and suggests such fatty acids as stearic acid, oleic acid, linoleic acid and conjugated linoleic acid. It is further stated that modified esters of glycerol and fatty acids such as triglycerides modified with fumaric acid or acrylic acid can also be used.
In U.S. Pat. No. 3,625,715 a fountain solution is disclosed that includes a polyethylene oxide with isopropyl alcohol, diethylene glycol, glycerine, a silicone-glycol copolymer surfactant and an antifoaming agent. It is stated that this solution may be substituted for conventional alcohol solutions without changes in operating procedures. The etch solution in U.S. Pat. No. 3,625,715 that is added to the water contains between 6 percent to 24 percent by weight isopropyl alcohol and between 3 to 9 percent by weight polyethylene oxide.
U.S. Pat. No. 3,877,372 discloses a fountain solution that includes ethylene glycol, monobutyl ether and at least one of hexylene glycol and ethylene glycol, a silicone glycol copolymer and a defoamer type surfactant. It is stated that isopropyl alcohol may be completely eliminated from this fountain solution.
The above attempts to replace the isopropyl alcohol with other materials have met with limited success either because the other materials are used in similar large quantities, i.e., from between 5 to 30 percent by volume or the amount of isopropyl alcohol has been reduced rather than being completely replaced; for example, a reduction in the isopropyl alcohol from 20 percent by volume to 10 percent by volume.
Isopropyl alcohol is a more volatile compound than water. Although isopropyl alcohol and water vapor pressures are not far different at ordinary temperatures, for instance the respective vapor pressures are about 30 and 18 mm of Hg at 20.degree. C., the heats of vaporization of water and isopropyl are considerably different. Isopropyl alcohol has a heat of vaporization of 164 callories/gm at its boiling point of 82.degree. C. while water has a heat of vaporization of 540 calories/gm at 100.degree. C. Thus in aqueous solutions, isopropyl alcohol migrates to and evaporates from surfaces much faster than water. Consequently, the isopropyl alcohol molecules in the air just above and at the liquid surface of its aqueous solutions must be at a higher concentration than in the bulk liquids. It is believed during the lithographic printing process employing isopropyl alcohol there is a concentrated water-containing isopropyl alcohol layer at all aqueous solution-air interfaces and at all ink-fountain solution admixture interfaces with air. Since isopropanol is miscible with lithographic inks, I also expect isopropanol-rich layers to be present at the air interface of the ink, when ink is admixed with isopropyl alcohol. In the presence of fountain solution containing isopropyl alcohol, I expect an isopropanol-rich layer of water is present at the ink-air interfaces.
If, as predicted here, an isopropanol-rich aqueous layer forms at all of the ink- and fountain-air interfaces, it is aqueous isopropanol that first comes into contact as the inked roller surfaces approach each other at the various nips--not ink and water surfaces. Wetting and intimate contact is thereby virtually assured even though the ink contains water. Since these isopropanol-rich aqueous layers readily wet either ink or water they can easily be squeezed back into their respective films or across to the opposite ink or water films at the nips. They do not need to be displaced. This allows intimate ink-to-ink contact where transfer of ink is required. And, it does not interfere with the required absence of ink transfer at ink-to-water nip areas of the printing plate.
Although not completely understood, I believe that the tendency for isopropanol to be at the air surface of all ink or water film portions of the plate and of the various rollers on the press account for its nearly universal practical acceptance as a fountain solution additive.
There is a need for a substitutive additive for isopropyl alcohol in a fountain solution that adds substantially the same advantageous features to the fountain solution such as the enhanced ink-water control without the disadvantageous features such as adverse toxic and safety properties and relatively high cost of the large volume of isopropyl alcohol that must be added to the fountain solution to be effective.